And Andre Parker, vice president of Hydro Ottawa, told local media in Montreal: "The latest information we have indicates that there was a break in one of the major US circuits, in Cleveland."

"Those circuits are all interconnected, and that sparked a
high-voltage chain reaction affecting Toronto, Detroit, Ottawa and New York," he added.

FirstEnergy utility company said alarm systems which might have alerted engineers to the failed lines were broken, but that functioning back-up systems had been in place.

The taskforce examining what went wrong will be jointly headed by US Energy Secretary Spencer Abraham, and the Canadian Minister of Natural Resources, Herb Dhaliwal.

US President George W Bush has said the blackout was a "wake-up call" to modernise an antiquated system.

The blackout affected eight US states and parts of eastern Canada, including in major cities like New York, Detroit, Cleveland, Toronto and Ottawa.

Power is now up and running in nearly all areas after nearly 30 hours without service, but problems in some areas were expected to run into Sunday and even next week.

Although the supply has been restored to much of Toronto, some residents in high-rise blocks of flats have complained they are still without power and water - and have been unable to flush their toilets since the crisis began.

Avoid 'frivolous use'

Ontario's Premier Ernie Eves has urged everyone to avoid what he called "frivolous use" of power.

"It is very aggravating to go by a lit-up billboard when others are going without electricity," he told reporters.